The invention is in general related to amplifier circuits and in particular to those having variable offset capability.
Amplifier circuits are used to amplify an input electrical signal to provide current and/or voltage gains or reductions. They may be used to amplify a single ended or a differential signal. A basic component of many amplifier circuits is the differential transistor pair used as the input stage of the amplifier. When the differential pair is used in conjunction with an output regenerative latch stage, a digital output signal (having one of two stable states) can be obtained that is an indication of a comparison between two single ended input signals or a determination of the magnitude of a differential signal. Due to the high gain and bistable nature of the latch stage, the output nodes of the latch stage are kept equalized until the correct information represented by the input signal can be expected to be present.
Most practical implementations of amplifier circuits suffer from manufacturing process-induced variations in the structure of the circuit elements, which cause an offset in the amplifier""s operation. This offset may be explained by, for instance, considering an amplifier that is designed to amplify a differential input signal. In the ideal case, the output of the amplifier would be zero volts if the input differential signal was zero volts. However, in practice, a zero voltage differential input signal often yields a small but nevertheless non-negligible output offset voltage.
Output offset may be corrected using a wide range of techniques known as offset cancellation techniques. In one such technique, the value of the input differential signal that actually yields a zero output voltage is measured and stored, and then is subsequently subtracted from each new input signal to thus cancel the offset of the amplifier.
To help reduce the inherent offset of the amplifier, the input differential transistor pair that receives the input signal is constructed using transistor matching techniques such that the pair is said to be balanced. The transistors of a balanced differential pair are typically replicates of each other in terms of size and as such are designed to mimic each other""s electrical characteristics. This also allows the amplifier to respond in the same manner to variations in the input signal that have opposite polarity but the same magnitude.